This invention relates to an electronic circuit for use in a single channel per carrier (often abbreviated to "SCPC") communication system and, in particular, to an electronic circuit within a transmitting section of the system that comprises a frequency controlling circuit.
In the manner known in the art, an SCPC communication system is used in a satellite communication system comprising a central station fixedly located at a predetermined terrestrial site, a satellite comprising a transponder, and at least one substation geographically spaced on the earth from the central station. In the SCPC communication system, a carrier wave of a single carrier frequency is assigned to a single communication channel. Such a single communication channel serves to transmit a communication signal, such as an audio signal, between the central station and the substation. In addition, a control channel is prepared to transmit a control signal between the central station and the substation and is common to both the central station and the substation. The control channel may be called a common signaling channel.
A frequency deviation of the carrier frequency inevitably takes place in the transponder of the satellite, as known in the art. A frequency control operation is carried out in the central station in order to compensate for the frequency deviation. In addition, a frequency deviation signal is transmitted from the central station as a control signal to the satellite through the common signaling channel.
The substation receives the communication signal through the satellite from the central station and derives information from the communication signal received on the communication channel allotted thereto. The substation also receives the frequency deviation signal from the satellite through the common signaling channel. The substation transmits information, corresponding to an input signal, to the satellite in the form of a burst at the carrier frequency, in accordance with a burst control signal produced in the manner known in the art. The burst control signal lasts during the period of time that the input signal is present. The substation has a frequency controlling unit for controlling the carrier frequency in response to the frequency deviation signal. Thus, the substation produces the burst, at the carrier frequency, under the control of the frequency controlling unit.
The frequency controlling unit comprises a local oscillator for producing a local oscillation signal, which has a variable frequency, as the carrier frequency.
In the frequency controlling unit, the carrier frequency of the local oscillation signal is continuously controlled, not only during absence of the input signal, but also during presence thereof. This means that the carrier frequency is widely varied during transmission of the burst, when the carrier frequency is controlled during presence of the input signal. A serious problem arises in the satellite. More specifically, the transponder of the satellite may have difficulty following the wide variation of the carrier frequency. As a result, the transponder may undesireably interrupt the communication channel allotted to the substation. In particular, the variation of the carrier frequency becomes large and serious when the local oscillator is a frequency synthesizer which generates a stepwise variable frequency.